When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment MZ is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, MZ, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is induced by the excited spins after the excitation field B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Magnetic resonance angiography (MRA) and, related imaging techniques, such as perfusion imaging, use the NMR phenomenon to produce images of the human vasculature or physiological performance related to the human vasculature. There are three main categories of techniques for achieving the desired contrast for the purpose of MR angiography. The first general category is typically referred to as contrast enhanced (CE) MRA. The second general category is phase contrast (PC) MRA. The third general category is time-of-flight (TOF) or tagging-based MRA.
To perform CE MRA, which is an invasive procedure in the sense that it requires the injection of a contrast agent, such as gadolinium, into the patient prior to the magnetic resonance (MR) angiogram to enhance the diagnostic capability of the MR angiogram. While CE MRA is a highly effective means for evaluating the vascular and physiological performance, for example, by studying perfusion, the technique suffers from several additional drawbacks. Specifically, an injection must be administered and, furthermore, the contrast agent that is injected carries its own drawbacks. In particular, contrast agents, generally, carry an appreciable financial cost and, furthermore, contrast agents, such as gadolinium, have recently been shown to be causative of a debilitating and potentially fatal disorder called nephrogenic systemic fibrosis (NSF). Further still, CE MRA may not provide accurate or sufficient hemodynamic information. Insufficient hemodynamic information, among other concerns, leaves the clinician without information about velocity and flow and, thus, it is not possible to determine if a stenosis is hemodynamically significant or to asses the perfusion in a clinically useful manner.
TOF imaging techniques do not require the use of a contrast agent. Contrary to CE-MRA, which relies on the administered contrast agent to provide an increase in measured MR signal, TOF MRA relies on the inflow of blood into an imaging volume to increase the signal intensity of the vasculature as compared to the stationary background tissues. This is achieved by the application of a number of RF excitation pulses to the imaging volume that cause the magnetization of the stationary background tissues to reach a saturation value. Since inflowing blood entering the imaging volume is not exposed to the same number of RF excitation, it will provide higher MR signal intensity than the background tissue. The differences between the signal intensity of the stationary background tissues and the inflowing blood thus provide a contrast mechanism exploited by TOF MRA. However, this process provides flow and velocity information only by approximation, which makes TOF good for qualitative analysis but not for quantitative analysis. Furthermore, the potential to require repeated excitations increases the amount of RF exposure delivered to the patient, which is regulated and controlled.
Phase contrast (PC) MRA relies on a change in the phase shifts of flowing protons in a region of interest to create an image. Spins that are moving along the direction of a magnetic field gradient receive a phase shift proportional to their velocity. Specifically, in a PC MRA pulse sequence, a bear minimum of two data sets with different amounts of flow sensitivity are acquired. For multi-dimensional PC MRA studies and images, three and four sets of data must be acquired. This is usually accomplished by applying gradient pairs, which sequentially dephase and then rephase spins during the sequence. In the most simplistic case of a one-dimensional (1D) velocity study, the first data set is acquired using a “flow-compensated” pulse sequence or a pulse sequence without sensitivity to flow. The second data set is acquired using a pulse sequence including velocity encoding designed to make the acquired data sensitive to flow in a particular direction. The amount of flow sensitivity is controlled by the strength of the bipolar gradient pairs used in the pulse sequence because stationary tissue undergoes no effective phase change after the application of the two gradients, whereas the different spatial localization of flowing blood is subjected to the variation of the bipolar gradient. Accordingly, moving spins experience a phase shift. The raw data from the two data sets are subtracted to yield images that illustrate the phase change, which is proportional to spatial velocity. To perform PC MRA pulse sequences, a substantial scan time is generally required, which increases with the desired number of dimensions desired in the acquired images, and the operator must set a velocity-encoding sensitivity, which varies unpredictably depending on a variety of clinical factors.
Therefore, it would be desirable to have a system and method for MR imaging, including those suitable for MRA applications, that does not suffer from the limitations addressed above.